Book, User, Address

book_user_address.rb

class Book
  include DataMapper::Resource

  property :id, Serial
  property :title, String
  property :author, String
  property :price, Decimal
end

class User
  include DataMapper::Resource

  property :id, Serial
  property :name, String

  property :address_country, String
  property :address_city, String

  validates_presence_of :name

  def address= address
    self.address_country = address.country
    self.address_city = address.city
  end

  def address
    Address.new(address_country, address_city)
  end
end

class Address < Struct.new(:country, :city)
end